The existing arsenic smelting methods are all normal pressure roasting reduction methods. So-called normal pressure means the air pressure in the furnace equal to the atmospheric pressure outside the furnace with connected air flow inside and outside the furnace. So-called reduction means that the arsenic extraction furnace must take As2O3 as raw material. That is, the minerals shall first be subject to oxidizing roasting to produce As2O3, which is then reduced to element arsenic under high temperature with electric reduction furnace and carbon. The shortcomings of such arsenic extraction method include: (1) The waste slag has arsenic oxides with big toxicity, which is unavoidable to cause pollution to the underground water and air. (2) The iron arsenate and arsenic oxides generated in the roasting process are left in the slag, leading to both big slag toxicity and low arsenic recovery. (3) SO2 concentration generated in the process of roasting cannot meet the requirement of acid making. The usual SO2 treatment by spraying lime water cannot reach the SO2 discharge standard. (4) In the process of mineral roasting till generating arsenic, each operating step cannot avoid the environment pollution.
To overcome the above disadvantages of using the As2O3 as raw material to produce element arsenic in the normal pressure reducing furnace, some research units made some small tests to directly extract element arsenic from arsenic concentrate by means of the vacuum process, such as the kilogram level test to remove arsenic from the cobalt ore in the existing technologies (China Non-ferrous Metal Journal, Book 4 Issue 1, 1993), which aimed at creating conditions for the next-step wet method extraction of element cobalt. The test theory was to enable thermal decomposition of the Co, Fe, Ni and As compound in the cobalt concentrate under vacuum conditions and separate out element arsenic. The experimental conditions were: residual pressure 6-10 Pa and temperature 1100-1200° C. But the experimental result had many problems: (1) Arsenic grade cannot meet the international requirement of 99% of arsenic and can only reach 76-92% of crude arsenic. Even the further distillation was also hard to reach the product requirement and involved high cost. (2) Since the smelting temperature was up to 1100-1200° C. and materials were under semi-molten state, it was difficult to discharge slag for application in industrial production. (3) The exhaust issue has not been solved. When arsenic vapor and vapor were generated in the furnace, they would cause the splash of molten materials and produce large quantity of dust polluting the arsenic product and hard to get qualified arsenic. (4) Arsenic content in the slag was up to 10-18%, which brought not only low arsenic recovery, but also the problem of further dearsenization requirement in the subsequent smelting sequence.
Another example is some medium and small tests made by means of existing horizontal type horizontal type rotary vacuum furnace to extract element arsenic from arsenopyrite ore, which still has many problems and has not been used for industrial product till now. Main problems are as follows: (1) The arsenic corrosion problem of rotary furnace has not been solved, leading to low furnace life and being not suitable for industrial production. (2) The furnace rotation generates large quantity of dust in the process of continuous stirring of materials, which seriously pollutes the product and is its second fatal weakness. (3) The exhaust problem has not been solved. Under high temperature, vapor generated from crystal water in materials directly enters the vacuum unit, often enables the impossible normal operation of vacuum pump and also leads to failure of vacuum solenoid valve. The requirement on vacuum degree cannot be guaranteed. Sometimes, the water accumulated in the vacuum pump leads to the oxidization of pump parts and rejection of vacuum pump. These accidents happened often lead to leakage of vacuum system and As2O3 pollution. (4) Due to continuous rotation of furnace shell, it is very difficult to measure the actual temperature in the rotary body. Plus, such furnace type integrates smelting chamber and crystallization chamber in the same furnace shell. It is more difficult to control temperature at connection between both chambers. (5) Deslagging and product stripping cannot be conducted at the same time. It must first conduct product stripping and then deslagging, which greatly extends the operating time. (6) Since the effective charging size of smelting chamber (material chamber) of horizontal type rotary furnace is small, and must be less than half the actual size of smelting chamber, otherwise, the materials will flow out of the vent hole (i.e. charging hole) upon rotation and continuously flow into the crystallization chamber and mix with the product. The above problems lead to the fact that the horizontal type rotary vacuum furnace cannot be used for industrial production.
Another example is given 100 g small tests made by arsenopyrite ore under vacuum conditions through thermal decomposition and extraction of element arsenic. The test ore charge is pure arsenopyrite ore. Firstly, the mineral is subject to cleaning to remove most impurities, and subject to leaching with the Iron(III) sulfate to remove FeS2 and other sulfides and get pure arsenopyrite ore as charging material. Although the qualified element arsenic can be got, it is easy to realize for small tests using pure arsenopyrite ore as charging material, and industrial production cannot meet such strict conditions. And the 100 g level vacuum furnace has its integrated smelting chamber, crystallization chamber and dust chamber. After furnace shutdown and temperature lowering, the particles of element arsenic are removed from the shell wall (crucible wall). Such tests can only show that the established fact of vacuum thermal decomposition and extraction of element arsenic.
Someone also made tests to adopt minor negative pressure operation in the furnace and enable thermal decomposition of arsenopyrite ore and extraction of element arsenic. So-called minor negative pressure is that the pressure difference between inside and outside the furnace is about 10 mm water columns. But the minor negative tests can also only show the established fact of thermal decomposition of arsenopyrite ore and extraction of element arsenic, and cannot eliminate the conditions of generating As2O3, far away from the industrial production.